bronze | alloy | Britannica.com Bronze Earth Bronze, alloy traditionally composed of copper and tin . Bronze is of exceptional historical interest and still finds wide applications. It was made before 3000 bc, though its use in artifacts did not become common until much later. The proportions of copper and tin varied widely (from 67 to 95 percent copper in surviving artifacts), but, by the Middle Ages in Europe, certain proportions were known to yield specific properties. An alloy described in an 11th-century Greek manuscript in the library of St. Mark’s, Venice, cites a proportion of one pound copper to two ounces of tin (8 to 1), approximately that used for bronze gunmetal in later times. Some modern bronzes contain no tin at all, substituting other metals such as aluminum, manganese , and even zinc . Bronze Lion Monument on the Burgplatz, in Braunschweig, Ger. H. Krause-Willemberg/ZEFA Bronze is harder than copper as a result of alloying that metal with tin or other metals. Bronze is also more fusible (i.e., more readily melted) and is hence easier to cast.

It is also harder than pure iron and far more resistant to corrosion . The substitution of iron for bronze in tools and weapons from about 1000 bc was the result of iron’s abundance compared to copper and tin rather than any inherent advantages of iron. copper processing: History Bell metal, characterized by its sonorous quality when struck, is a bronze with a high tin content of 20–25 percent. Statuary bronze, with a tin content of less than 10 percent and an admixture of zinc and lead , is technically a brass . Bronze is improved in hardness and strength by the addition of a small amount of phosphorus; phosphor bronze may contain 1 or 2 percent phosphorus in the ingot and a mere trace after casting, but its strength is nonetheless enhanced for such applications as pump plungers, valves, and bushings. Also useful in mechanical engineering are manganese bronzes , in which there may be little or no tin but considerable amounts of zinc and up to 4.5 percent manganese.

Aluminum bronzes, containing up to 16 percent aluminum and small amounts of other metals such as iron or nickel , are especially strong and corrosion-resistant; they are cast or wrought into pipe fittings, pumps, gears, ship propellers, and turbine blades. Similar Topics What Is Bronze? - Chemistry Definition Updated October 16, 2016. Question: What Is Bronze? Bronze is one of the earliest metals known to man. Here is a look at the chemical composition and properties of bronze. Answer: Bronze is defined as an alloy made of copper and another metal, usually tin . Compositions vary, but most modern bronze is 88% copper and 12% tin. Bronze may also contain manganese, aluminum, nickel, phosphorus, silicon, arsenic, or zinc. Although at one time bronze was any alloy consisting of copper with tin and brass was an alloy of copper with zinc, modern usage has blurred the lines between brass and bronze. Now, copper alloys generally are called brass, with bronze sometimes considered a type of brass . To avoid confusion, museums and historical texts typically use the inclusive term "copper alloy". In science and engineering, bronze and brass are defined according to their element composition.

Bronze Properties Bronze usually is a golden hard, brittle metal. The properties depend on the specific composition of the alloy as well as how it has been processed. continue reading below our video 10 Best Universities in the United States Here are some typical characteristics: Highly ductile . Bronze exhibits low friction against other metals. Many bronze alloys display the unusual property of expanding a small amount when solidifying from a liquid into a solid. For sculpture casting, this is desirable, as it helps to fill a mold. Brittle, but less so than cast iron. Upon exposure to air, bronze oxidizes, but only on its outer layer. This patina consists of copper oxide, which eventually becomes copper carbonate. The oxide layer protects the interior metal from further corrosion. However, if chlorides are present (as from sea water), copper chlorides form, which can cause "bronze disease" -- condition in which corrosion works through the metal and destroys it. Unlike steel, striking bronze against a hard surface won't generate sparks. This makes bronze useful for metal used around flammable or explosive materials. Origin of Bronze The Bronze Age is the name given to the time period when bronze was the hardest metal that was widely used.

This was the 4th millennium BC about the time of the city of Sumer in the Near East. The bronze age in China and India occurred at roughly the same time. Even during the Bronze Age, there were a few items crafted from meteoritic iron, but the smelting of iron was uncommon. The Bronze Age was followed by the Iron Age, starting around 1300 BC. Even during the Iron Age, bronze was widely used. Uses of Bronze Bronze is used in architecture for structural and design elements, for bearings because of its friction properties, and as phosphor bronze in musical instruments, electrical contacts, and ship propellers. Aluminum bronze is used to make machine tools and some bearings. Bronze wool is used instead of steel wool in woodworking because it doesn't discolor oak. Bronze has been used to make coins. Most "copper" coins are actually bronze, consisting of copper with 4% tin and 1% zinc. Bronze has been used since ancient times to make sculptures.

The Assyrian king Sennacherib (706-681 BC) claimed to be the first person to cast huge bronze sculptures using two-part molds, although the lost-wax method was used to cast sculptures long before this time. Difference Between Copper, Brass and Bronze | Metal Supermarkets Difference Between Copper, Brass and Bronze Difference Between Copper, Brass and Bronze Category: Brass , Bronze , Copper Posted: April 21, 2015 Copper, Brass and Bronze, otherwise known as the “Red Metals”, may look the same initially but are actually quite different. Copper Copper is used in a wide range of products due to its excellent electrical and thermal conductivity, good strength, good formability and resistance to corrosion. Pipe and pipe fittings are commonly manufactured from these metals due to their corrosion resistance. They can be readily soldered and brazed, and many can be welded by various gas, arc and resistance methods. They can be polished and buffed to almost any desired texture and luster. There are grades of unalloyed Copper, and they can vary in the amount of impurities that are contained. Oxygen-free copper grades are used specifically in functions where high conductivity and ductility are needed.

One of the most important properties of copper is its ability to fight bacteria. After extensive antimicrobial testing by the Environmental Protection Agency, it was found that 355 copper alloys, including many brasses, were found to kill more than 99.9% of bacteria within two hours of contact. Normal tarnishing was found not to impair antimicrobial effectiveness. Applications Copper was one of the earliest metals discovered. The Greeks and Romans made it into tools or adornments, and there are even historical details showing the application of copper to sterilize wounds and purify drinking water. Today it is most commonly found in electrical materials such as wiring because of its ability to effectively conduct electricity. Brass Brass is mainly an alloy that consists of copper with zinc added. Brasses can have varying amounts of zinc or other elements added. These varying mixtures produce a wide range of properties and variation in color. Increased amounts of zinc provide the material with improved strength and ductility. Brass can range in color from red to yellow depending on the amount of zinc added to the alloy. If the zinc content of the brass ranges from 32% to 39%, it will have increased hot-working abilities but the cold-working will be limited.

If the brass contains over 39% zinc (example – Muntz Metal), it will have a higher strength and lower ductility (at room temperature). Applications Brass is commonly used for decorative purposes primarily because of its resemblance to gold. It is also a commonly used to make musical instruments due to its high workability and durability. Other Brass Alloys Tin Brass This is an alloy that contains copper, zinc and tin. This alloy group would include admiralty brass, naval brass and free machining brass. The tin has been added to inhibit dezincification (the leaching of zinc from brass alloys) in many environments. This group has low sensitivity to dezincification, moderate strength, high atmospheric and aqueous corrosion resistance and excellent electrical conductivity. They possess good hot forgeability and good cold formability. These alloys are typically used to make fasteners, marine hardware, screw machine parts, pump shafts and corrosion-resistant mechanical products. Bronze Bronze is an alloy that consists primarily of copper with the addition of other ingredients. In most cases the ingredient added is typically tin, but arsenic, phosphorus, aluminum, manganese, and silicon can also be used to produce different properties in the material. All of these ingredients produce an alloy much harder than copper alone.

Bronze is characterized by its dull-gold color. You can also tell the difference between bronze and brass because bronze will have faint rings on its surface. Applications Bronze is used in the construction of sculptures, musical instruments and medals, and in industrial applications such as bushings and bearings, where its low metal on metal friction is an advantage. Bronze also has nautical applications because of its resistance to corrosion. Other Bronze Alloys Phosphor Bronze (or Tin Bronze) This alloy typically has a tin content ranging from 0.5% to 1.0%, and a phosphorous range of 0.01% to 0.35%. These alloys are notable for their toughness, strength, low coefficient of friction, high fatigue resistance, and fine grain. The tin content increases the corrosion resistance and tensile strength, while the phosphorous content increases the wear resistance and stiffness. Some typical end uses for this product would be electrical products, bellows, springs, washers, corrosion resistant equipment. Aluminum Bronze This has an aluminum content range of 6% – 12%, an iron content of 6% (max), and a nickel content of 6% (max).

These combined additives provide increased strength, combined with excellent resistance to corrosion and wear. This material is commonly used in the manufacturing of marine hardware, sleeve bearings and pumps or valves that handle corrosive fluids. Silicon Bronze This is an alloy that can cover both brass and bronze (red silicon brasses and red silicon bronzes). They typically contain 20% zinc and 6% silicon. Red brass has high strength and corrosion resistance and is commonly used for valve stems. Red bronze is very similar but it has lower concentrations of zinc. It is commonly used in the manufacturing of pump and valve components. Nickel Brass (or Nickel Silver) This is an alloy that contains copper, nickel and zinc. The nickel gives the material an almost silver appearance. This material has moderate strength and fairly good corrosion resistance. This material is typically used to make musical instruments, food and beverage equipment, optical equipment, and other items where the aesthetics are an important factor. Copper Nickel (or Cupronickel) This is an alloy that can contain anywhere from 2% to 30% nickel. This material has a very high corrosion-resistance and has thermal stability.

This material also exhibits a very high tolerance to corrosion cracking under stress and oxidation in a steam or moist air environment. Higher nickel content in this material will have improved corrosion resistance in seawater, and resistance to marine biological fouling. This material is typically used in making electronic products, marine equipment, valves, pumps and ship hulls. Metal Supermarkets – A Premier Supplier of Copper, Brass and Bronze Metal Supermarkets sells a wide variety of copper , brass and bronze shapes and grades. No minimum order quantity and cut to your size. Buy online or visit one of our 75+ locations in North America. Share: Bronze BRONZE Bronze is a metal alloy consisting primarily of copper, usually with tin as the main additive, but sometimes with other elements such as phosphorus, manganese, aluminium, or silicon. It is hard and brittle, and it was particularly significant in antiquity, giving its name to the Bronze Age. The word Bronze is believed to be cognate with the Italian: bronzo and German: brunst, perhaps ultimately taken from the Persian word birinj ("bronze") or possibly from the Latin name of the city of Brindisi (aes Brundusinum-Pliny).

History The discovery of bronze enabled people to create better metal objects than was previously possible. Tools, weapons, armor, and various building materials, like decorative tiles, made of bronze were harder and more durable than their stone and copper ("Chalcolithic") predecessors. Initially bronze was made out of copper and arsenic to form arsenic bronze. It was only later that tin was used, becoming the sole type of bronze in the late 3rd millennium BC. Tin bronze was superior over arsenic bronze in that the alloying process itself could more easily be controlled (as tin was available as a metal) and the alloy was stronger and easier to cast. Also, unlike arsenic, tin is not toxic. The earliest tin-alloy bronzes date to the late 4th millennium BC in Susa (Iran) and some ancient sites in Luristan (Iran) and Mesopotamia (Iraq). Copper and tin ores are rarely found together (exceptions include one ancient site in Thailand and one in Iran), so serious bronze work has always involved trade. In Europe, the major source for tin was Great Britain's deposits of ore in Cornwall, which were traded as far as Phoenicia in the Eastern Mediterranean.

Though bronze is generally harder than wrought iron, with Vickers hardness of 60–258 vs. 30–80, the Bronze Age gave way to the Iron Age; this happened because iron was easier to find. Bronze was still used during the Iron Age, but, for many purposes, the weaker wrought iron was found to be sufficiently strong. Archaeologists suspect that a serious disruption of the tin trade precipitated the transition. The population migrations around 1200–1100 BC reduced the shipping of tin around the Mediterranean (and from Great Britain), limiting supplies and raising prices. As ironworking improved, iron became cheaper; and as cultures advanced from wrought iron to forged iron, they learned how to make steel, which is stronger than bronze and holds a sharper edge longer. Properties Bronze is considerably less brittle than iron. Typically bronze only oxidizes superficially; once a copper oxide (eventually becoming copper carbonate) layer is formed, the underlying metal is protected from further corrosion. However, if copper chlorides are formed, a corrosion-mode called "bronze disease" will eventually completely destroy it. Copper-based alloys have lower melting points than steel or iron, and are more readily produced from their constituent metals.

They are generally about 10 percent heavier than steel, although alloys using aluminium or silicon may be slightly less dense. Bronzes are softer and weaker than steel - bronze springs, for example, are less stiff (and so store less energy) for the same bulk. Bronze resists corrosion (especially seawater corrosion) and metal fatigue more than steel and is also a better conductor of heat and electricity than most steels. The cost of copper-base alloys is generally higher than that of steels but lower than that of nickel-base alloys. Copper and its alloys have a huge variety of uses that reflect their versatile physical, mechanical, and chemical properties. Some common examples are the high electrical conductivity of pure copper, the excellent deep drawing qualities of cartridge case brass, the low-friction properties of bearing bronze, the resonant qualities of bell bronze, and the resistance to corrosion by sea water of several bronze alloys. The melting point of Bronze varies depending on the actual ratio of the alloy components and is about 950 °C. Uses Bronze was especially suitable for use in boat and ship fittings prior to the wide employment of stainless steel owing to its combination of toughness and resistance to salt water corrosion. Bronze is still commonly used in ship propellers and submerged bearings.

In the twentieth century, silicon was introduced as the primary alloying element, creating an alloy with wide application in industry and the major form used in contemporary statuary. Aluminium is also used for the structural metal aluminium bronze. It is also widely used for cast bronze sculpture. Many common bronze alloys have the unusual and very desirable property of expanding slightly just before they set, thus filling in the finest details of a mould. Bronze parts are tough and typically used for bearings, clips, electrical connectors and springs. Spring bronze weatherstripping comes in rolls of thin sheets and is nailed or stapled to wood windows and doors. There are two types, flat and v-strip. It has been used for hundreds of years because it has low friction, seals well and is long lasting. It is used in building restoration and custom construction. Bronze also has very little metal-on-metal friction, which made it invaluable for the building of cannon where iron cannonballs would otherwise stick in the barrel.[citation needed] It is still widely used today for springs, bearings, bushings, automobile transmission pilot bearings, and similar fittings, and is particularly common in the bearings of small electric motors. Phosphor bronze is particularly suited to precision-grade bearings and springs.

It is also used in guitar and piano strings. Bronze is typically 88% copper and 12% tin. Alpha bronze consists of the alpha solid solution of tin in copper. Alpha bronze alloys of 4–5% tin are used to make coins, springs, turbines and blades. Commercial bronze (90% copper and 10% zinc) and Architectural bronze (57% Copper, 3% Lead, 40% Zinc) are actually brass alloys because they contain zinc as the main alloying ingredient. They are commonly used in architectural applications. Unlike steel, bronze struck against a hard surface will not generate sparks, so it (along with beryllium copper) is used to make hammers, mallets, wrenches and other durable tools to be used in explosive atmospheres or in the presence of flammable vapours. Bronze statues Indian Hindu artisans from the period of the Chola empire in Tamil Nadu, used bronze to create intricate statues via the lost wax casting method with ornate detailing depicting the Gods of Hinduism mostly, but also the lifestyle of the period. The art form survives to this day, with many silpis, craftsmen, working in the areas of Swamimalai and Chennai.

In antiquity other cultures also produced works of high art using bronze. For example: in Africa the bronze heads of the Kingdom of Benin, in Europe; Grecian bronzes typically of figures from Greek mythology, in east Asia; Chinese bronzes of the Shang and Zhou dynasty — more often ceremonial vessels but including some figurine examples. Bronze continues into modern times as one of the materials of choice for monumental statuary. Musical instruments Bronze is the most popular metal for top-quality bells, particularly bell metal, which is about 23% tin. Nearly all professional cymbals are made from a bronze alloy. The alloy used in drum kit cymbal bronze is unique in the desired balance of durability and timbre. According to a legend (on the Zildjian cymbals website), in 1623, an Armenian man in Turkey named Avedis Zildjian, an alchemist, was attempting to form base metals into gold. Upon dropping an ingot on the ground, he was amazed at how well it rang. He was given the title Zildjian ("son of cymbal maker") by the Turkish Sultan.

Today, the Avedis Zildjian Corporation is the largest maker of cymbals in the world. Modern cymbals consist of several types of bronze, the most common being B20 bronze, which is roughly 20% tin, 80% copper, with traces of silver. Zildjian and Sabian use this alloy for their professional lines. A Swiss company, Paiste, uses a softer B8 bronze which is made from 8% tin and 92% copper in nearly all of their cymbals. Zildjian and Sabian use this metal too, in their budget priced cymbals. As the tin content in a bell or cymbal rises, the timbre drops. As well as B8 and B20, Meinl Percussion uses B10 (10% tin) and B12 (12% tin) alloys for cymbals, which have timbres roughly between B8 and B20. Bronze is also used for the windings of steel strings of various stringed instruments such as the double bass, piano, harpsichord, and the guitar, replacing former gut and nylon strings.

Bronze strings are commonly reserved on pianoforte for the lower pitch tones, as they possess a superior sustain quality to that of high-tensile steel. Bronzes of various metallurgical properties are widely used in struck idiophones around the world, notably in South East Asia, and most famously for the Javanese gamelan and other glockenspiel-like musical instruments. The earliest bronze archeological finds in Indonesia date from 1–2 BCE, including flat plates probably suspended and struck by a wooden or bone mallet. Some companies are now making saxophones from phosphor bronze (3.5 to 10% tin and up to 1% phosphorus content). Brass vs Bronze - Difference and Comparison | Diffen Brass vs. Bronze Diffen › Science › Chemistry Brass and Bronze are metal alloys used extensively in everyday objects. While brass is an alloy of copper and zinc, bronze is an alloy consisting mainly of copper, combined most often with tin, but at times also with other metals. Owing to their properties, these two alloys have various uses. Comparison chart Brass versus Bronze comparison chart Brass Bronze Properties Higher malleability than zinc or copper. Low melting point (900 c); flows when melted.

Combinations of iron, aluminum, silicon & manganese make brass corrosion resistant. Susceptible to stress cracking when exposed to ammonia. Not as hard as steel. Hard and brittle. Melts at 950 centigrade but depends on amount of tin present. Bronze resists corrosion (especially seawater corrosion) and metal fatigue more than steel and is also a better conductor of heat and electricity than most steels. Composition Brass is any alloy of copper and zinc. Bronze is a metal alloy consisting primarily of copper, usually with tin as the main additive, but sometimes with other elements such as phosphorus, manganese, aluminum, or silicon. Color Muted yellow, somewhat similar to gold, but duller. Reddish brown. Uses Decorative; Low-friction applications (locks, gears, doorknobs, ammunition, valves); Plumbing/electronics; Musical instruments for acoustic properties; Zippers & uses where it's important to negate spark(fittings & tools around explosive gas). Used in boat and ship fittings, propellers and submerged bearings because of resistance to salt water corrosion. Widely used for cast bronze sculpture; Bearings, clips, electrical connectors and springs; For top-quality bells and cymbals.

History Brass was first known to exist in about 500 BC. Bronze dates to about 3500 BC. Contents: Brass vs Bronze Brass Statue Properties Brass has higher malleability than zinc or copper. It has a a low melting point (900 centigrade ) and flows when melted making it easy to cast in molds. Combinations of iron, aluminum, silicon and manganese make brass wear and tear and corrosion resistant. Susceptible to stress cracking when exposed to ammonia. Bronze Sculpture Bronze is hard and brittle. It melts at a slightly higher temperature at 950 centigrade, but this depends on the amount of tin present in the alloy. Bronze resists corrosion (especially seawater corrosion) and metal fatigue more than steel and is also a better conductor of heat and electricity than most steels. The composition of both alloys depends on the particular use. For instance, Cartridge brass contained 30% zinc and was used to make cartridges for firearms. Naval brasses had up to 39.7% Zinc and were used in various applications on ships.

Bismuth bronze is a bronze alloy with a composition of 52 parts copper, 30 parts nickel, 12 parts zinc, 5 parts lead, and 1 part bismuth. It is able to hold a good polish and so is sometimes used in light reflectors and mirrors. Uses Musical instruments: The malleability and acoustic properties of brass have made it the metal of choice for brass musical instruments such as the trombone, tuba, trumpet, cornet, euphonium, tenor horn, and the French horn. Even though the saxophone is classified as a woodwind instrument and the harmonica is a free reed aerophone, both are also often made from brass. Bronze is the most popular metal for top-quality bells, particularly bell metal, which is about 23% tin. Nearly all professional cymbals are made from a bronze alloy. The alloy used in drum kit cymbal bronze is unique in the desired balance of durability and timbre. Phosphor bronze is also used in guitar and piano strings. Sculptures and statues: Brass is often used for decoration, statues and coins for its bright gold -like appearance and its relative resistance to tarnishing.

Many common bronze alloys have the unusual and very desirable property of expanding slightly just before they set, thus filling in the finest details of a mould so widely used for cast bronze sculpture. Machine parts: Brass is used for applications where low friction is required such as locks, gears, bearings, doorknobs, ammunition, and valves. It is used for plumbing and electrical applications. Bronze is ideally used today for springs, bearings, bushings, automobile transmission pilot bearings, and similar fittings, and is particularly common in the bearings of small electric motors. Phosphor bronze is particularly suited to precision -grade bearings and springs. Bronze was especially suitable for use in boat and ship fittings prior to the wide employment of stainless steel owing to its combination of toughness and resistance to salt water corrosion. Bronze is still commonly used in ship propellers and submerged bearings History The first known existence of bronze dates to about 3500 BC and the Sumerians and lends its name to the Bronze age. The discovery of bronze enabled people to create better metal objects than before. Tools, weapons , armor, and various building materials, like decorative tiles, made of bronze were harder and more durable than their stone and copper.

Brass came later dates back to about 500 BC. Zinc is practically never found naturally in its pure state, but people had realized that copper smelted with calamine -- a zinc ore -- produced a golden-colored tarnish-resistant metal that was useful for all sorts of things due in part to its low melting point and malleability. The zinc itself is not seen but is released from the calamine ore by heating and combines immediately with copper. References Bronze: Characteristics, Uses And Problems Bronze: Characteristics, Uses And Problems Procedure code: Outdoor Sculpture Manual - Center For Public Buildings Division: 08/05/2016 Introduction The following guidelines provide general information on the characteristics and common uses of bronze and identify typical problems associated with the material. See also "Checklist for Inspecting Bronze Failures". References Gayle, M., Look, D. and Waite, J. Metals in America's Historic Buildings: Uses and Preservation Treatments. Washington, DC: Department of the Interior, National Park Service, 1992. Preservation Science. "Preventing Galvanic Corrosion. By Choosing the Right Materials". Web. 2008. Weaver, M.

Conserving Buildings: Guide to Techniques and Materials (1st Edition). New York: Wiley, 1997. Zahner, L. W. Architectural Metal Surfaces. New York: Wiley, 2004. Introduction Bronze is an alloy of copper which can vary widely in its composition. It is often used where a material harder than copper is required, where strength and corrosion resistance is required and for ornamental purposes. The variations in bronze (both in proportion and elemental composition) can significantly affect its weathering characteristics. "True" bronze is a combination of approximately 90% copper (Cu) and 10% tin (Sn), however there are three major classes or types of "bronzes" used in sculpture and construction. They are: Statuary Bronze - approximately 97% copper (Cu), 2% tin (Sn) and 1% zinc (Zn); this composition is the closest to "true" bronze. Architectural Bronze - actually more of a "leaded brass", this composition is commonly composed of approximately 57% copper (Cu), 40% zinc (Zn) and 3% lead (Pb).

Commercial Bronze - composed of approximately 90% copper (Cu) and 10% zinc (Zn). Traditionally, a copper alloy which contains zinc is a "brass"; a copper alloy which contains tin (not exceeding 11%) is a "bronze". Bronze composition may vary significantly however, and contemporary bronzes are typically copper alloys which may contain silicon (Si), manganese (Mn), aluminum (Al), zinc (Zn) and other elements, with or without tin (Sn). In its "raw" state, bronze is a semi-pink or salmon-colored metal; however it is rarely seen in its pure state. Bronze usually exhibits some patination or corrosion so that its color normally ranges from lime green to dark brown. Exposed bronze undergoes continuous change and progresses through several predictable "stages" of oxidation and corrosion. The stages of bronze corrosion vary in duration and time of onset, based on many factors, including: Composition of the bronze Acid rains Bird droppings Problems may be classified into two broad categories: 1) Natural or inherent problems based on the characteristics of the material and the conditions of the exposure, and 2) Vandalism and human- induced problems.

Although there is some overlap between the two categories, the inherent material deterioration problems generally occur gradually over long periods of time, at predictable rates and require appropriate routine or preventive maintenance to control. Conversely, many human induced problems, (especially vandalism), are random in occurrence; can produce catastrophic results; are difficult to prevent, and require emergency action to mitigate. Some human induced problems, however, are predictable and occur routinely. Natural or Inherent Problems Bronze, like cast iron, is a manufactured product. Copper is extracted from natural ores and alloyed with tin to create a metal which does not exist in nature. Many of the inherent problems relate to the normal physical process of the bronze "returning to nature", i.e. to the most stable states of its components. Additionally, most outdoor bronze is erected with a foundry applied patina of some type. The actual surface patina could be one of dozens of different composites as a result of the foundry applied finishes. Each of these finishes may react differently with the environment and result in different corrosion types and rates. Regardless of which finish exists, the bronze will begin the deterioration process described below, where the surface will be subjected to the alteration of the patina through oxidation and sulfurization.

Patinated and protected surfaces will resist the effects of exposure more than bare metal; therefore, such pieces will maintain their original appearance longer and exhibit changes more slowly. Corrosion Corrosion of one form or another is the chief cause of the deterioration of metals, including statuary and architectural bronze. The degree of corrosion which occurs, and the corrosion by-products which result, are affected by several factors including bronze composition or formulation, environmental conditions and adjacent materials. While the composition of bronze does affect the rate of corrosion, it has been generally recognized that composition is one of the least significant factors in bronze deterioration. The existence of chemicals in the atmosphere, such as chlorine, sulfur, and nitrogen oxides, in the presence of moisture, is the most significant cause of bronze deterioration. There are numerous causes and symptoms of corrosion, including: Uniform O xidation or Corrosion: Corrosion attacks the metal surface evenly. Pitting: Attacks the metal surface in localized areas. Selective Attack: When a metal is not homogenous throughout, certain areas may be attacked in preference to others. Erosion: When a corrosion-resistant oxide layer is removed and the bare metal beneath corrodes.

Oxygen Cell Corrosion (or Atmospheric Corrosion): The most common form of corrosion; Moisture containing environmental gases (carbon dioxide, oxygen, sulfur compounds, soot, fly ash, etc.) produces chemical corrosion on the metal. Galvanic Corrosion: The increased corrosion of a metal due to its contact with another metal, or in some cases, the same metal. Galvanic corrosion causes extensive deterioration to the attacked metal(s), and in turn the corrosion products stain and streak the adjacent surfaces. It is an electrolytic reaction. For this to occur, there must be an anode (negatively charged area), a cathode (positively charged area), and an electrolyte (conducting medium). The electrolyte can be rainwater, condensation, acid, alkali, or a salt. The formation of an anode and a cathode may occur due to the presence of impurities, difference in work hardening, or local differences of oxygen concentration on the surface. Stress Corrosion Cracking: Attacks areas in a metal which were stressed during metal working.

Humidity, temperature and condensation: Affect the rate of corrosion; in a marine environment, aerosols can deposit chloride and other salts which will accelerate the rate of atmospheric corrosion. The bronze corrosion process goes through five predictable stages. The specific results of each stage can differ due to combinations of atmospheric elements, bronze composition, patination, and other protective treatments such as waxing, oiling or lacquering.The five stages are: Induction is when normal oxidation takes place, normally producing the dark brown copper oxide film which can be a protective barrier against future pollutants. The actual film composition is dependent upon the type and concentration of pollutants in the atmosphere, upon the duration of exposure, and upon the relative degree and duration of wetness on the surface. High concentrations of sulfides in the atmosphere can dramatically alter the result of stage 1, producing less protective, even potentially damaging films. The rate of oxidation can also have an effect on long term durability of the surface finish; oxides formed over longer time periods seem much more resistant to deterioration. The conversion of the topmost metallic surface to copper sulfate normally begins to occur on surfaces with the most severe exposure, such as horizontal surfaces.

Oxygen deprivation and deposition of particulates and moisture create a catalytic situation where electrolytic reactions occur. (This is the same principle as a battery, where the charged ions move from a positive to a negative pole.) The visual symptom of this phase is the formation of thin, light green patches on the more exposed areas. Run-off streaking and scab formation occurs at a slower rate than the two previous stages but the consequences are significant. Copper sulfates and sulfides may have been formed during the earlier stages, yet the degree of solubility of these compounds may vary widely. It is during Stage 3 that the familiar streaking and uneven discoloration may occur due to differential weathering of the corrosion by-products. This erosion can continue until uneven blackish areas or island- like scabs are present on the surface. Pitting may spread around the black scab formation; the pitting can also continue to spread below what appears to be a stable surface. Pitting is generally caused and accelerated by microscopic particles of chlorides deposited from the air, and if chlorides are present below a crust or a barrier coating, the corrosion can continue unchecked and invisible to casual observation.

Complete conversion of all exposed surfaces to the bright blue-green copper sulfate is the final stage of corrosion. The result is the familiar solid green bronze with the lime- green color and a matte texture. This condition is sometimes misperceived as the desirable end condition, but it is actually a phase of active corrosion. Oxidation Unprotected areas of raw bronze will oxidize, or combine with oxygen present in the air, resulting in a thin film of copper oxide along the surface of the exposed bronze. The resulting appearance is a flat, dark brown surface. The most common example to which most users can relate is the process of oxidation of a copper penny. The specular (shiny) finish of a new penny is familiar, as is the shift to the dark, red-brown finish as the surfaces oxidize over time. This normal process of oxidation is a form of corrosion. The resultant oxide film is less reactive than raw bronze and forms a stable, protective barrier with a greatly reduced rate of oxidation. Sulfurization Bronze also reacts with many atmospheric pollutants, especially sulfur compounds, which are normally found in the atmosphere as sulfur dioxide and hydrogen sulfide. Both are produced in industrial manufacturing processes.

Concentrations of these gasses are generally greater in or near urban and industrial areas; therefore higher rates of corrosion can normally be expected in such areas. The initial symptom of sulfurization is the appearance of patches of light green primarily on exposed surfaces. This usually begins on horizontal surfaces which receive the greatest exposure to rains and water run-off. A general layer of surface corrosion can eventually spread over the entire metallic surface, resulting in an overall bright green surface. The uniform green surface is often accepted by the general public, and others, as protective and the normal state of bronze. This is a misconception, and one which has probably resulted in the public acceptance of appearances which are actually symptoms of corrosion and deterioration. The sulfides and sulfates will continue to form in the presence of moisture and atmospheric sulfur compounds. The presence of green corrosion products on the bronze is always an indication of active corrosion. The pattern and result of this process will vary based upon several environmental factors such as wind, rain, pollutants, patina, and the nature of previous corrosion. Differential weathering due to winds, rain and surface orientation can result in uneven corrosion with patterns of green streaking on a dark blackish surface.

The process of sulfurization is complicated by two factors, both of which result in aesthetically unacceptable appearances; appearances which are generally perceived as neglect and deterioration. Uneven black and green streaking of bronzes is one of the most disfiguring problems which can occur with bronze. Random dark (black) and light (green) streaks follow the contours downward, resulting in distracting visual patterns with no relationship to the form or texture of the surface of the work. The artistic details which give form and definition to the bronze become extremely obscured by streaking which results from two phenomena: Differential solubility of the corrosion products, and Electrochemical processes between the dark (black) and light (green) areas. The streaking of bronze indicates a differential corrosion of the bronze which will be permanently disfiguring. Two different surface corrosion products are dissolving at significantly different rates. The geological analogy is the formation of canyons by the erosion of the land surface. Where such corrosion has already occurred, conservation techniques are likely to be required. Early indications of streaking should be given serious attention in the inspection process, and called to the attention of the Regional Historic Preservation Officer (RHPO) at the earliest possible time.

Bronze Disease Bronze disease is the result of exposure to chlorine compounds which can come from any saline source, such as contact with saline soils, atmospheric pollutants or airborne salt spray near bodies of salt water. The chlorine reacts with the copper in bronze to form copper chloride. The primary symptom is pitting, and the process can proceed unchecked below apparently sound patinas, or protective coatings. The copper chloride is relatively unstable and the only way to arrest the continuing corrosion is the complete removal of the chlorides using electrochemical methods. All such methods of chloride removal are advanced conservation techniques requiring the employment of a skilled professional. Core Migration Bronze is cast in a foundry process which consists of the pouring of molten bronze into a mould containing a central core. Frequently this core material is gypsum or plaster of Paris, and occasionally portions of the core are left inside the casting. It is possible for the core material to migrate through the casting wall over time and appear on the exterior surface of the bronze. The removal and repair of core migration problems is not a maintenance procedure and will require an "existing conditions analysis" supporting a proposed conservation treatment. The RHPO should be notified of the problem following its identification.

The most common symptom is the appearance of whitish spots, which gradually enlarge, in the bronze surface. Pitting Corrosion of bronze, unlike that of natural stones, is in part an electro-chemical phenomenon. Points of negative electrical potential called cathodes and points of positive potential called anodes form on the bronze. In the presence of moisture, the corrosion process is driven by an electrical differential between the two points. This process can occur at a highly accelerated rate. An electric potential can develop between both large and small areas. Atmospheric pollutants, especially chlorides, can be deposited on the surface of bronze. Tiny "islands" of corrosion can form, rapidly eroding/converting away the bronze metal and resulting in tiny voids or pits in the surface of the bronze. Pits may begin small and increase in size due to the continued electrochemical action and deposition within the pits. This may continue as long as moisture is present. Pitting may be pinpoint or broad, as in patterns of deep etching created by differential erosion. (Also see: Bronze Disease) Bird Droppings Bird, or other animal, droppings may collect on the surface of bronze and (because of the acidic nature) may accelerate localized corrosion and deterioration.

Droppings can also build up in sheltered areas, providing concentrations of damaging chemical agents of deterioration. Galvanic Corrosion Galvanic corrosion, also known as dissimilar metal corrosion, occurs when two dissimilar metals are brought into contact with one another. One of the metals will corrode, and the other will remain intact. As an example, if bronze is brought into contact with iron, the iron will frequently begin to corrode. Galvanic corrosion is caused by an electric potential between two dissimilar metals in the presence of water or moisture, where the water's electrolytes allow the flow of metallic ions from the more active metal, or the anode, to the more noble metal, or the cathode. The movement of these metallic ions represents a physical loss of metal from the metal being corroded. It can continue until the source metal is completely gone. Below, thirteen construction metals are ranked according to their susceptibility to corrosion, from most to least susceptible, or from active to noble. This type of ordered list is called a Galvanic Series chart. The rate of the transfer of iron from the passive to the active metal is determined by the difference in electrode potential between the two metals.

Therefore, the farther apart two metals are in the list below, the more likely the active metal (higher on the list) is to corrode. Zinc Graphite Gold Galvanic corrosion typically occurs where dissimilar metals are used as connectors or parts of a building's armature. It can be stopped by replacing the more active metal with a more noble metal such as stainless steel. When two dissimilar metals must be in contact with one another, the risk of corrosion can be substantially reduced by applying a coating to both of the materials but especially to the noble metal, or applying a sacrificial metallic coating that is more active than both of the metals. The relative mass or sizes of the two metals in contact will also determine the rate at which galvanic corrosion occurs. As an example, in a bronze plaque with iron bolts, the bolts would corrode rapidly, but an iron plaque with bronze or copper bolts would exhibit a much lower, almost negligible, amount of galvanic corrosion as a result of its contact with the bolts. Therefore, bolts and other fasteners should be made of more noble metals where possible.

Erosion Erosion or "wearing away" of metal from the surface may be due to natural or environmental factors, or due to man-induced factors such as excessive handling or rubbing. Erosion due to human contact is by far the most serious problem, but erosion can occur due to the abrasive action of wind-driven pollutants. Natural erosion will be a slow process and one which is, therefore, difficult to detect. It will be most obvious on outdoor bronze or in exposed locations. Industrial settings and areas where there are higher concentrations of airborne particulates, which can act as abrasives, also offer the possibility for higher rates of erosion. Natural, wind-driven abrasion will be generally so slow that it will be most apparent when comparing different exposures/orientations of bronze which has been in service for long periods. The differential loss of detail between protected and exposed surfaces will begin to be apparent over many years. Examination for this differential weathering should be part of any inspection. Vandalism or Human-Induced Problems Mechanical Deterioration (Purely Physical Processes) Abrasion: Causes removal of the protective metal surface.

Some metals such as zinc are relatively soft and therefore vulnerable to abrasion damage, especially in areas similar to roof valleys where the metal can be worn paper-thin. Fatigue: Failure of metal that has been repeatedly stressed beyond its elastic limit, due to failure to provide necessary allowances for thermal expansion and contraction caused by temperature differences. Creep: The permanent distortion of a soft metal which has been stretched due to its own weight. Thin areas of the metal will be among the first to fail. Can be found in lead sculptures which have inadequate or corroded internal armature. Heat: Usually in the form of fire, will cause many metals to become plastic, distort, and fail. Distortion: Permanent deformation or failure may occur when a metal is overloaded beyond its yield point because of increased live or dead loads, thermal stresses, or structural modifications altering a stress regime. Connection Failure Chemical and mechanical processes can cause the breakdown or reduced effectiveness of structural metal fixings such as bolts, rivets, and pins. Stress failure is often a contributor to breakdown situations. Iron connections which are water traps are particularly susceptible. Most bronze corrosion can be characterized as "general" or "uniform" and "pitting", with occasional signs of selective attack.

Galvanic corrosion appears mostly in connection with pins, bolts, and replacement parts in different metal. Erosion is apparent most often in bronzes in fountains. Stress corrosion is less apparent in bronze than in brass, but could be a factor in some cases in bronze sculptures. Bronze Alloys | National Bronze & Metals, Inc. Bronze Alloys Bronze Metal Alloys Since the establishment of our foundry in Lorain, Ohio in 1998, NBM Metals has been the leading manufacturer of continuous cast bronzes in North America. Our production consists of bronze bars in rounds, hollows, rectangles, squares, hexagons, and made to order shapes for customer-specific applications. We utilize both vertical and horizontal computer-controlled casting processes at our foundry, and have the capability to continuous cast bronze as large as sixteen inches in diameter (both solids and hollows). We are also recognized throughout North America for casting the largest wear plates in both high-leaded tin bronzes and aluminum bronzes. We also maintain inventories of different sized centrifugal cast bronze bars in both high-leaded tin bronze and aluminum bronze bars .

Bars up to thirty inches in diameter are readily available for next-day shipment to accommodate urgent breakdowns and repair work. NBM Metals has a long, family heritage of manufacturing wrought bronzes that no other U.S. company can compare. Contact our staff for more information or request a quote for your material and get a quick response. Brass Alloys

Question: Bronze is an alloy of copper and which other metal?
Answer: Tin